Trichomonas vaginalis is protozoan parasite that causes trichomoniasis, one of the most common and treatable of the sexually transmitted diseases. Trichomonas vaginalis is a relatively delicate pear-shaped trophozoite that is typically 7 to 23 μm long by 5 to 12 μm wide. The organism has four anterior flagella and a fifth forming the outer edge of a short undulating membrane. The anterior flagella propels the organism through liquid in a jerky, rapid fashion, sometimes causing the organism to rotate as it moves. Trichomonas vaginalis divides by binary fission in the urogenital tract of those infected. The organism is clear, uncolored, or slightly grey in appearance under the microscope. A slender rod, the axostyle, extends the length of the body and protrudes posteriorly. The nucleus is near-anterior and appears well-defined, containing many chromatin granules. The appearance of T. vaginalis is very similar to that of other trichomonads, such as Trichomonas tenax, although only T. vaginalis is found in genitourinary tract infections.
Worldwide, T. vaginalis infects approximately 180 million people per year, usually by direct person-to-person contact, making it the most common sexually transmitted disease (STD) agent. In the United States, it is believed that T. vaginalis infects an estimated 5 million people annually. Despite its prevalence and geographic distribution, T. vaginalis has not been the focus of intensive study. Indeed, it is not even listed as a “reportable disease” by the U.S. Centers for Disease Control, and there are no active control or prevention programs. Recent reports, however, suggest growing public health interest in this pathogen.
Infections in women are known to cause vaginitis, urethritis, and cervicitis. Severe infections are accompanied by a foamy, yellowish-green discharge with a foul odor, and small hemorrhagic lesions may also be present in the genitourinary tract. Complications include premature labor, low-birth weight offspring, premature rupture of membranes, and post-abortion and post-hysterectomy infection. An association with pelvic inflammatory disease, tubal infertility, and cervical cancer have been reported. Trichomonas vaginalis has also been implicated as a co-factor in the transmission of HIV and other STD agents. The organism can also be passed to neonates during passage through the birth canal.
In men, symptoms of trichomoniasis include urethral discharge, urethral stricture, epididymitis, the urge to urinate, and a burning sensation with urination. In both men and women, infections with T vaginalis are usually asymptomatic and self-limiting. It is estimated that, in women, 10-50% of T. vaginalis infections are asymptomatic, with the proportion in men probably being even higher. That said, with many women the infection becomes symptomatic and chronic, with periods of relief in response to therapy. Recurrence may be caused by re-infection from an asymptomatic sexual partner, or by failure of the standard course of therapy (a regimen of the antibiotic metronidazole). And while T. vaginalis infections almost always occur in the genitourinary tract, on rare occasions they occur at ecotopic sites, and the parasite may be recovered from other areas of a patient's body.
As a result of suboptimal comparative laboratory methods and a focus on other STD sources, studies of T. vaginalis have often substantially underestimated the prevalence of infection. Despite this, levels of infection typically have been high, with reported overall prevalences ranging from 3-58%, with an unweighted average across studies of 21% (Cu-Uvin et al. Clin. Infect. Dis. (2002) 34(10):1406-11). In studies that presented information on race/ethnicity, T. vaginalis infection rates have been reported to be highest among African-Americans (Sorvillo et al. Emerg. Infect. Dis. (2001) 7(6):927-32). The following chart illustrates the trend reported by Sorvillo et al., with regard to the prevalence of infection in terms of the percentage of patients infected with trichomoniasis, chlamydia, and/or gonorrhea at certain health clinics in Baltimore, Md. (B) and in New York, N.Y. (NY).
PatientChlamydiaGonorrheaYearNumberCityTrichomoniasis (%)(%)(%)1996213NY519 51994372NY277 219941404NY2015No Data1992279B2621141990-94677NY226 1
Following exposure, the incubation period ranges from about 5 to 10 days, although periods as short as 1 day to as many as 28 days have been reported. If diagnosed, T. vaginalis infections can be readily treated by orally administered antibiotics.
Given its relative prevalence and association with other STDs, there is increasing interest in effectively diagnosing trichomoniasis. Conventional diagnostic methods for detecting T. vaginalis, however, are based on direct examination, “wet mount” microscopy, or cell culture, each of which has its own shortcomings. With regard to direct patient examination, other infections mimic the appearance and odor of the vaginal discharge. Accordingly, laboratory techniques such as microscopy, antibody detection, and cell culture are often used. While it is possible to detect T. vaginalis using a “wet mount” prepared by mixing vaginal secretions with saline on a slide and examining the slide under a microscope for the presence of organisms having the characteristic size, shape, and motility of T. vaginalis, the sensitivity of such methods depends highly on the skill and experience of the microscopist, as well as the time spent transporting specimen to a laboratory. Wet mount diagnosis has been found to be only 35-80% as sensitive as other methods, such as cell culture, in detecting the presence of T. vaginalis. Other direct methods, such as fluorescent antibody detection and enzyme-linked immunoassays, have also been developed, as has a non-amplified, DNA probe-based method (Affirm, Becton Dickinson), although their sensitivities, as compared to cell culture, range from 70-90%. For these reasons, cell culture is considered the current “gold standard” for clinical detection of T. vaginalis. Due to its relatively delicate nature, however, the organism is technically challenging, and typically requires up to 7 days for maximum sensitivity. Even then, the sensitivity of cell culture methods is estimated to be only about 85-95% due to problems associated with time lapses between sample recovery and culture inoculation, maintaining proper incubation conditions, visualizing low numbers of the organism and/or the motility of the protozoa.
Given the human health implications of trichomoniasis and relative inability of existing clinical laboratory methods to selectively and sensitively detect T. vaginalis from a test sample, a need clearly exists for a sensitive and specific assay which can be used to determine the presence of T. vaginalis in a particular sample of biological material.